Numerous applications in science, industry and health care require detectors that can perform measurements on incident radiation, e.g., determine the time and position at which the radiation was incident. A position sensitive device (PSD) originally described by J. T. Wallmark, “A new semiconductor photocell using lateral photoeffect”, Proc. IRE, vol. 45, no. 4, pp. 474-483, April 1957 (see also U.S. Pat. No. 3,028,050 to J. T. Wallmark) has become a useful tool for such purposes, since the PSD is specifically designed to yield information about the time of incidence of the radiation, its energy and location of its centroid.
The most successful types of PSDs thus far are based on solid-state devices with a photosensitive, reverse biased p-n semiconductor junction. Salient aspects of such p-n semiconductor junction PSDs are described in U.S. Pat. No. 4,749,849 to Hoeberechts et al. and in U.S. Pat. No. 4,877,951 to Muro. Several recent PSD implementations use avalanche photodiodes with internal gain as described, for example, by Karplus et al. in U.S. Pat. Nos. 6,781,133 and 6,998,619.
Some applications, e.g., in health care, require measurement of short wavelength radiation. To perform such measurements it is known to extend the useful range of a PSD by interposing a fluorescent material such as a scintillator crystal between the high-energy radiation and the PSD. The scintillator converts the high-energy radiation, e.g., X-rays or gamma rays into lower energy radiation. The PSD then performs its measurements on the low energy secondary radiation, e.g., in the visible or infra-red range to determine time of incidence, total energy and centroid location. Particular interest in the scintillator based PSD approach is found in the fields of Positron Emission Tomography (PET) or single photon emission computed tomography (SPECT), where most solutions rely on less compact apparatus, e.g., devices employing photomultiplier tubes (PMTs). Some examples of using avalanche photodiode arrays and position sensitive avalanche photodiodes (PSAPDs) for detecting gamma rays using scintillation crystals can be found in K. S. Shah et al., “Large-area apds and monolithic apd arrays”, IEEE Trans. Nucl. Sci., Vol. 48, No. 6, December 2001, pp. 2352-2356; K. S. Shah et al., “Position-sensitive avalanche photodiodes for gamma ray imaging”, IEEE Trans. Nucl. Sci., Vol. 49, No. 4, Part 1, August 2002, pp. 1687-1692, and N. Zhang et al., “RF Transformer Coupled Multiplexing Circuits for APD PET Detectors”, Nuclear Science, IEEE Transactions, Vol. 53, Issue 5, Part 1, October 2006, pp. 2570-2577.
Manufacturing approaches for commercially viable, large area and high-gain avalanche photodiodes that can be used in PSAPDs are known in the art. These typically involve producing an array of discrete pixels or position sensitivity within one large pixel on a monolithic device. For more information, the reader is referred to U.S. Pat. No. 5,021,854 to Huth, U.S. Pat. Nos. 5,757,057 and 6,111,299 to Dabrowski, U.S. Pat. No. 5,500,376 to Ishaque and Gramsch et al. “High density avalanche photodiode array”, Proc. SPIE Vol. 2022, October 1993, pp. 111-119.
One of the drawbacks to reading out an individual array of many discrete solid-state PSD pixels or a system built from many monolithic solid-state PSDs involves the large number of outputs per device and their relatively low amplification capabilities as compared to a PMT. In fact, PMTs easily lend themselves to multiplexed readout, as described, e.g., by S. Siegel et al., “Simple charge division readouts for imaging scintillator arrays using a multi-channel pmt”, IEEE Trans. Nucl. Sci., Vol. 43, No. 3, June 1996, pp. 1634-1641 (part 2) and by P. D. Olcott et al., “Compact readout electronics for position sensitive photomultiplier tubes”, IEEE Trans. Nucl. Sci., Vol. 52, No. 1, February 2005, pp. 21-27. Meanwhile, multiplexing of solid-state PSDs such as PSAPDs represents an unsolved challenge.